This invention relates to an LCD projector for enlarging and projecting an image on a liquid crystal panel and the like, and more particularly to an LCD projector in which a utilization efficiency of light emitted from a light source is enhanced to increase an optical output from the liquid crystal panel.
There have conventionally been put on the market an apparatus for enlarging and projecting an image on a liquid crystal panel by means of a light source such as a metal halide lamp, in which apparatus the light emitted from the light source is converged on the liquid crystal panel via a mirror or the like so that the image can be projected onto a screen through a projection lens. There are known two types of apparatus, that is, a single liquid crystal panel type (referred to as "a single panel type", hereinafter) and a three panel type in which three liquid crystal panels are used and color is separated and mixed by means of a dichroic mirror or the like. In both types, the light emerging from the projection lens is converted into polarized wave and, in general, only the vertically-polarized wave is allowed to appear on the screen surface in most cases.
Generally, a single panel type LCD projector comprises, as shown in FIG. 10, a projection lens 41, a field lens 42, a liquid crystal material 43, an incoming-side polarizer 44 and an outgoing-side polarizer 45, the incoming-side polarizer 44 cutting off half of the light.
A typical example of the three panel type LCD projector is disclosed in Japanese Patent Unexamined Publication No. 63-15225, for example. In this apparatus as well, half of the light (P-polarized light or S-polarized light) is cut off by the incoming-side polarizer as useless light. As for the means for utilizing both of the separated P- and S-polarized lights by converting one of the P- and S-polarized lights into the other of the P- and S-polarized lights, there has been proposed a polarization converting device shown in FIG. 8, for example. Further, as disclosed in Japanese Patent Unexamined Publication No. 5-323236, it has conventionally been proposed to improve brightness by converting only into the P-polarized light to project onto three liquid crystal panels by making use of the principle of the polarization converting device (FIG. 9).
FIG. 8 chiefly shows a polarization converting device section. A beam of randomly polarized light 26 emitted from a light source 25 is divided into two linearly-polarized beams, that is, into a P-polarized beam 27 and an S-polarized beam 28 by means of a polarizing beam splitter 29, and then converted into P-polarized beams 35, 36 by means of a few prisms 31, 32, 33 and 34, so that a lot of light is converted on a liquid crystal panel 37.
A polarization converting device section shown in FIG. 9 also converges lots of light on a liquid crystal panel surface by polarizing the light only in one direction by means of a polarizing beam splitter 37 and two prisms 38, 39.
In the LCD projector of either three or single panel type, the incoming-side polarizer and the outgoing-side polarizer are arranged before and behind the liquid crystal material, and the angle of rotary polarization of each picture cell is controlled in response to the voltage applied to the liquid crystal material.
According to systems in FIGS. 8 and 9 and the like, the light flux from the light source is converted only into the P-polarized light before arriving at the incoming-side polarizer to double a quantity of usable light which has been half of the light from the light source in the prior art. Therefore, it was possible to gain brightness, but many prisms should be combined for achievement, which resulted in a huge cost.
According to the system shown in FIG. 9, P-polarized lights from two directions are made incident on the panel by means of a combination of prisms and two kinds of lenses put on them, and therefore there were serious problems on positional accuracy of component parts, cost and the like.
Moreover, there have been proposed no systems for ensuring a uniform brightness of the liquid crystal panel surface even at the periphery thereof.